1. Field of the Invention
This invention relates to a gripper for gripping and removing tubes from tube bundles. It is particularly useful in removing sample heat-exchanger tubes from the secondary side of a nuclear steam generator.
2. Description of the Prior Art
Tube grippers for gripping and pulling tubes out of the tube bundles of heat exchangers are known in the prior art. Typically, such tube grippers include an expandable collet having a plurality of gripping threads which is insertable into the mouth of the tube to be pulled. The actuator mechanism of such grippers is usually a tapered expander member which may be forcefully engaged into the mouth of the expandable collet. The tapered expander member is connected to a rod which is slidably engaged through the mouth of the collet through a centrally disposed bore in the collet. When a tensile force is applied to the rod, the tapered expander member is drawn up into the mouth of the collet. The wedging forces that the tapered expander member applies to the mouth of the collet spreads the walls of the collet outwardly so that its outside threads grippingly engage the inner walls of the tube. After the tube has been gripped, a hydraulic ram or other pulling means applies a tensile force to the entire gripper in order to pull the tube out of its respective tube bundle.
In some of these prior art mechanisms, the tensile force applied by the hydraulic ram is used both to spread the walls of the expandable collet into gripping contact with the inner walls of the tube, as well as to withdraw the tube from the tube bundle. However, such mechanisms require some sort of collet-retaining mechanism, such as an arrangement of hydraulically-operated, telescoping sleeves, to simultaneously push the expandable collet into the mouth of the tube while the tensile force pulls on the tapered expander member; otherwise, the tensile force applied by the hydraulic ram would merely pull the expandable collet out of the mouth of the tube instead of expanding the collet into gripping engagement with the inner walls of the tube.
While such prior art tube grippers provide good results when the mouth of the tube is in a freely accessible location (such as the tubesheet in the primary side of a nuclear steam generator), such grippers are difficult to use in areas where the mouth of the tube is in a less accessible location, such as the secondary side of a nuclear steam generator. To fully appreciate the difficulties associated with using such prior art tube grippers in the secondary side of such a generator, some general background is necessary concerning both the structure and maintenance procedures associated with nuclear steam generators.
Nuclear steam generators generally include a primary side through which hot, radioactive water from the reactor core is admitted into a plurality of heat exchange tubes which may be U-shaped. Such generators further include a secondary side which houses and spaces these tubes and circulates a flow of non-radioactive water therethrough so that non-radioactive steam may be generated from the energy output of the reactor core. The tubes in the secondary side are densely arranged in a tube bundle. The tubes of the steam generator transfer heat from the radioactive water flowing through their inside surfaces to the non-radioactive water flowing over their outside surfaces. To facilitate this heat transfer, a plurality of horizontally disposed baffle plates and support plates are mounted throughout the secondary side of the steam generator in order to uniformly space the heat-exchanging tubes from one another. Each of these plates includes a plurality of uniformly-spaced openings (which may be bores) through which the tubes extend. The openings of the plates are arranged in registry with one another in order that the legs of the U-shaped heat-exchange tubes may be arranged parallel to one another.
Over a period of time, corrosive sludge deposits may accumulate around the legs of the heat exchange tubes in the sections of these tubes which are surrounded by the tubesheet which separates the primary side of the steam generator from the secondary side, and the bores of the support plates, which are located in the secondary side of the steam generator. In order to determine the extent to which these sludge deposits attack the walls of the heat exchange tubes in these areas, samples of these tubes must be periodically cut and withdrawn either through the primary side of the generator from the tubesheet, or from the secondary side of the generator.
When the sample tube is taken from the primary side of the generator through the tubesheet, the mouth of the tube is freely accessible since it is flush against the side of the tubesheet which faces the primary side of the generator. However, the mouths of such sample tubes in the secondary side of the generator are not nearly as accessible. In taking such secondary-side samples, it is first necessary to cut the greater part of the bight portion of the U-shaped tube being sampled, thereby opening the upper ends of the two legs of the U-shaped tube. Each of the legs is then cut again with an inner-diameter tube cutter in order to free it from the tubesheet. Unfortunately, the upper, open ends of these sample tubes are thickly surrounded by the bight portions of the other U-shaped tubes which are not being sampled. If one attempts to use a prior art tube gripper to grip and pull one of these sample tubes, two problems immediately arise. First, the bulkiness and size of the collet-retaining mechanism may make it impossible to position the gripper deep inside the tube bundle between the thickly arranged heat exchange tubes so that the expandable collet is properly positioned inside the mouth of the sample tube. Even if the tube gripper is sufficiently small in cross-sectional area so it may be inserted between these adjacent tubes, it is almost always necessary to use some sort of extension mechanism to position the gripper and to connect it to the hydraulic ram. However, when this is done, proper and reliable actuation of the expanding member becomes the second problem. As previously mentioned, the proper actuation of prior art grippers requires some sort of mechanism for simultaneously applying a collet-retaining force while a tensile force is applied to the tapered expanding member in order to draw it up into engagement with the expanding collet. An extension mechanism which incorporates such a collet-retaining mechanism (like the previously mentioned pair of telescoping sleeves, or a rod and cylinder arrangement) is difficult and expensive to machine, and difficult and time-consuming to operate. Additionally, such a design does not provide an extension mechanism which is easily length-adjustable (i.e., easily built in segments until the desired length is obtained).
Accordingly, there is a need for a tube gripper which may be easily inserted into a sample tube in a crowded tube bundle and operated by means of a simple, rod-like extension which may be conveniently adjusted to a desired length. Further, it would be desirable if the gripper itself were simple in structure, reliable in operation, and capable of quickly gripping and pulling sample tubes from a nuclear steam generator so that the time spent by maintenance personnel in the radioactive environment of the nuclear steam generator is minimal.